The present invention relates to a handheld optoelectronic measuring device, in particular a sighting device, including an electronic magnetic compass for determining the azimuthal orientation of the measuring device, and a method for determining an expected accuracy of the azimuthal orientation of a measuring device including a magnetic compass. According to the present invention, by means of the measuring device, it is automatically detected if the expected accuracy is too low, in particular if a new compensation must be carried out, and at least one warning message is given to the user.
Such measuring devices are used for determining coordinates of distant objects, for example in object recording and data collection for geographical information systems (GIS). Such measuring devices may also be in the form of portable sighting devices which are used in particular for determinations of coordinates of military target objects, as described, for example, in U.S. Pat. No. 7,325,320 B2.
Such a determination of target coordinates requires the relative coordinates between the measuring device and the target object. For this purpose, the sighting device is aligned with the target object, and the azimuthal orientation and the zenith orientation of the sighting device relative to the Earth are then determined. The angle values determined can then be provided, together with a typical accuracy value in each case, at a data interface of the sighting device for transmission to a firing control post. From the firing control post, the firing activity can then be introduced via a firing unit into an area associated with the transmitted target coordinates.
With regard to the achievable accuracy of the target coordinates to be determined, the magnetic compass is the critical component. On the basis of the transmitted accuracy value of the azimuthal orientation, it is possible on the one hand to assess the effect of firing activity to be introduced on the target object, and on the other hand, to assess the probability of collateral damage. In the case of a substantial difference between the effective and the specified typical accuracy value, this assessment may be incorrect.
Even with an electronic magnetic compass, considerable caution is still advisable in the determination of azimuthal orientations, although the components of the magnetic and gravitational fields are measurable as such with sufficient device accuracy. As disclosed in U.S. Pat. No. 4,949,089, today, it is possible to take into account the declination of the Earth's magnetic field from geographic north virtually automatically by means of the magnetic variation compensation implemented in military GPS receivers. Since, however, in addition to the Earth's magnetic field, which is the medium for information relating to the north direction, the measured magnetic field as a rule includes stray magnetic fields superposed thereon, the azimuthal orientation relative to geographic north can nevertheless often only be determined with very limited accuracy and reliability, which may be several times the accuracy of the device itself.
These stray magnetic fields comprise stationary stray fields associated with the measuring location and stray fields which relate to the device itself and are due to electrical currents and magnetically hard and magnetically soft materials of the device in which the magnetic compass is installed. Stationary stray fields can moreover be divided into stray fields of regional scale and stray fields of local scale.
Stray fields of regional scale, so-called anomalies of the Earth's magnetic field, are as a rule due to natural interfering effects, for example extensive deposits of iron ore. When considered on a local scale, these stray fields are homogeneous and result locally in a constant azimuthal error comparable to the declination of the Earth's magnetic field.
On the other hand, stray fields of local scale are due to man-made objects, for example railway tracks, water pipes, overhead lines, pipelines, or structures made of steel and reinforced concrete. Quasi-stationary objects, such as parked vehicles or weapons systems brought into position, also cause stray magnetic fields of local scale. When considered on a local scale, these stray fields are inhomogeneous and cause varying azimuthal errors, even within measuring spaces with dimensions of meters, which may also vanish in some locations.
As disclosed in DE 196 09 762 C1, fixed stray fields of a device including an electronic magnetic compass which has sensors for the three-dimensional measurement of a magnetic field and a gravitational field can be arithmetically compensated by means of a vector equation when determining azimuthal orientations of the device. The parameters of the vector equation must be determined beforehand by means of an optimization method. This optimization method is based on values of a more or less rigidly specified sequence of measurements of the magnetic and gravitational fields at a measuring location. In the case of each of these measurements, the device is oriented differently in space. In this way, however, stationary stray magnetic fields can be neither compensated nor detected at the measuring location.
U.S. Pat. No. 6,539,639 B2 discloses a method in which the accuracy in the determination of an azimuthal orientation using a magnetic compass is said to be monitorable. Since such a magnetic compass has sensors for the three-dimensional measurement of the magnetic and gravitational fields, the values of the horizontal and vertical field strengths of the magnetic and gravitational fields are also obtained in the determination of an azimuthal orientation. The method is based here on a comparison of these values with stored values of the horizontal and vertical field strengths from the past in light of a specified threshold value.
The stored values of the horizontal and vertical field strengths may be attributable on the one hand to a determination of the parameters, comparable with DE 196 09 762 C1, at a different measuring location, or, on the other hand, may be obtained by averaging horizontal and vertical field strengths of past determinations of the azimuthal orientation. Since, however, there is no direct relationship between a change in the horizontal and vertical field strengths between different measuring locations and the occurrence of azimuthal errors and, on the other hand, significant azimuthal errors may occur even in the case of similar horizontal and vertical field strengths at different measuring locations, false alarms occur again and again in the case of such monitoring, or necessary warnings are not given. In addition, such a warning also contains only the information that differences of horizontal and/or vertical field strengths of a certain magnitude were determined over a certain period.
It would be helpful if information were provided to the user warning about current interfering effects. Such an error or accuracy indication with respect to the azimuth measurement is not mentioned in any of the aforementioned documents.